National Stormwater Calculator

Additional Information

Tool to help control runoff and promote the natural movement of water

EPA’s National Stormwater Calculator (SWC) is a software application that estimates the annual amount of rainwater and frequency of runoff from a specific site. Estimates are based on local soil conditions, land cover, and historic rainfall records. It is designed to be used by anyone interested in reducing runoff from a property, including site developers, landscape architects, urban planners, and homeowners.

The SWC accesses several national databases that provide soil, topography, rainfall, and evaporation information for a chosen site. The user supplies information about the site’s land cover and selects low impact development (LID) controls they would like to use. The LID controls include seven green infrastructure practices.

Green infrastructure practices, which are the LID controls used in the SWC, promote the natural movement of water, instead of allowing it to wash into streets and down storm drains. Having less water runoff into storm drains and roadways can help prevent contamination of waterways, infrastructure degradation, flooding, and overwhelming of treatment plants. This allows stormwater to be used as a resource rather than a waste product, and can add aesthetic and economic value to a community. The following green infrastructure practices are included in the SWC:

Hydrology Analysis. The SWC allows users to analyze site hydrology for small- to medium-sized (less than 12 acres) locations within the United States, including Puerto Rico, using LID controls. It estimates the amount of stormwater runoff generated from a site under different development and control scenarios over a long-term period of historical rainfall.

Cost Module. An LID cost estimation module within the application allows planners and managers to evaluate LID controls based on comparison of regional and national project planning level cost estimates (capital and average annual maintenance) and predicted LID control performance. Cost estimation is accomplished based on user-identified size configuration of the LID control infrastructure and other key project and site-specific variables. This includes whether the project is being applied as part of new development or redevelopment and if there are existing site constraints.

Climate Scenarios.The SWC allows users to consider how runoff may vary based on historical weather and potential future climate conditions. To better inform decisions, it is recommended that users develop a range of results with various assumptions about model inputs. Please check with local authorities about whether and how use of these tools may support local stormwater management goals.

Note: The SWC uses the Storm Water Management Model (SWMM) as its computational engine. SWMM is a well-established, EPA developed model that has seen continuous use and periodic updates for 40 years. Its hydrology component uses physically meaningful parameters making it especially well-suited for application on a nation-wide scale. SWMM is set up and run in the background without requiring any involvement of the user.

The SWC is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. Its primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. It can be used to answer questions such as the following:

What is the largest daily rainfall amount that can be captured by a site in pre-development, current, or post-development condition?

To what degree will rainfall from storms of different magnitudes be captured on site?

What mix of LID controls can be deployed to meet a given stormwater retention target?

How well will LID controls perform under future meteorological projections made by global climate change models?

What are the relative planning level costs (capital and maintenance) differences for various mixes of LID controls?

The SWC is available as a mobile web application that can be used on desktop devices and mobile devices, such as smartphones and tablets, and is compatible with all operating systems. It works best with the following browsers: Microsoft Edge, Google Chrome, Mozilla Firefox, and Apple Safari. The SWC is also available as a Windows-based desktop program that runs on any version of Microsoft Windows with version 4 or higher of the .Net Framework installed. Both versions of the SWC require an internet connection.

Disclaimer: Any mention of trade names, manufacturers, or products does not imply an endorsement by EPA. EPA and its employees do not endorse any commercial products, services, or enterprises.

Documentation

Related EPA Resources

Green Infrastructure Modeling Toolkit can be used as a teaching tool and as a quick reference resource for use by planners and developers when making green infrastructure implementation decisions for stormwater management.